X-ray and gamma-ray scintillators and detector screens incorporating same

ABSTRACT

An X-ray or gamma-ray scintillator comprising, on a substrate, an alkaline halide layer of fibrous structure having the needle form, with the needles perpendicular to the surface of the substrate, the scintillator being associated, in detector screens of image-intensifier tubes for X-ray or gamma-ray, with a photosensitive layer enshrouding said needles.

United StatesTatent 1191 Driard et a1.

[ 1 Get. 30, 1973 X-RAY AND GAMMA-RAY SCENTILLATORS AND DETECTOR SCREENS INCORPORATING SAME [75] Inventors: Bertrand M. Driard; Lucien F.

Guyot; Guy Roziere, all of Paris, France [73] Assignee: Thomas-CSF, Paris, France 22 Filed: Dec. 22, 1970 [21] Appl. No.: 100,564

[30] Foreign Application Priority Data Dec. 30, 1969 France 6945437 52 US. 01. 117/33.s R, 117/219, 117/223, 117/224, 250/71 R, 250/715 R, 250/80, 250/213 51 Int. Cl C09k l/06 [58] Field of Search 117/335 R, 33.5 E, 117/335 CP, 106 R, 219, 223, 224; 250/71 [56] References Cited UNITED STATES PATENTS 3,515,587 6/1970 Letter 117/223 X 3,275,827 9/1966 Lind 250/71 X 2,752,521 6/1956 lvey 250/80 X 3,558,893 1/1971 Ball 250/71.5 X

Primary Examiner-Alfred L. Leavitt Assistant Examiner-Caleb Weston Attorney-Cushman, Darby & Cushman [57] ABSTRACT An X ray or gamma-ray scintillator comprising, on a substrate, an alkaline halide layer of fibrous structure having the needle form, with the needles perpendicular to the surface of the substrate, the scintillator being associated, in detector screens of imageintensifier tubes for X-ray or gamma-ray, with a photo-sensitive layer enshrouding said needles.

8 Claims, 3 Drawing Figures X-RAY AND GAMMA-RAY SCINTILLATORS AND DETECTOR SCREENS INCORPORATING SAME The present invention relates to the design of X-ray and gamma-ray scintillators.

A particularly important application of these is in image-intensifier tubes for X-ray and gamma-ray use.

This is why, in order to illustrate the invention, a tube of this kind has been selected, purely by way of a nonlimitative example since it is far from being the only possible application of the scintillators in accordance with the invention, as will become apparent hereinafter.

At this juncture, we will briefly recapitulate on the principle operation of these tubes An X-ray or gamma-ray image-intensifier tube comprises, within an evacuated envelope, two screens which are generally situated at the two ends of the envelope, one being the detector screen and the other the display screen.

On the path of the incident beam of radiation, there are disposed in succession the object and the imageintensifier tube, the detector screen of the tube being located opposite the object. This screen receives the radiation which has passed through the object.

The detector screen comprises, at the object side, a layer which is referred to as the luminescent or scintillation layer and capable of emitting photons within the luminous radiation range, this being referred to as luminous photons and this emission taking place under the effect of the radiation received from the object and a photo-sensitive layer which emits electrons under the effect of said photons, this layer being referred to as a photocathode.

The means required are in addition provided in the tube in order to accelerate and focus onto the display screen, the electrons emitted by the photocathode in this fashion an image of the object is produced by bombardment of said screen.

A variety of solutions have been put forward in the past for the design of detector screens of this kind, amongst which some provided for a central transparent substrate ateither side of which the luminescent and photo-sensitive layers were located, and some provided for the direct deposition of the photo-sensitive layer upon the luminescent layer.

Whatever the design which is used and the nature of the material employed, ZnS, ZnCdS, INa, ICs, Ca W difficulties are encountered as far as adhesion of the luminescent layer to the substrate and its stability under the effect of expansion, are concerned.

All things being equal, these difficulties are of course the more serious the greater the thickness of the scintillater.

However, it is well known in the context of the design of these tubes, that it is desirable to increase the absorption of X-ray or gamma-ray photons and, consequently, for a given material, to increase the thickness of the luminescent layer in order to improve the tube efficiency, that is to say its quantic efficiency, i.e., the luminosity of the final image produced by the display screen. It goes without saying that this thickness cannot be increased beyond a certain limit without the risk of interfering with the spatial resolution of the image if this thickness is too great, the image of a point on the display screen will not be a strictly spot image because of the diffusion of the light photons through the thickness of the scintillator.

The object of the present invention is to design scintillators of alkaline halides, which are thicker than those of the prior art but nevertheless exhibit good adhesion to their substrate and good stability in operation.

The invention is applicable, amongst other things, to the composite detector screens of X-ray and gammaray image-intensifier tubes as hereinbefore described, which comprise an alkaline halide luminescent layer for example of cesium iodide, potassium iodide, sodium with one or more activators such as thallium or sodium deposited by vaporisation under vacuum on a substrate, and a photo-sensitive layer of one of the known materials such as complex compounds of antimony and alkaline metals, cesium, potassium, sodium, rubidium.

The invention is based upon the observation, stemming from work carried out in the laboratories of the applicants, that the adhesion of the scintillator layer to its substrate is improved if said scintillator layer has a fibrous structure obtained by successive operations of vaporisation under vacuum of the constituent material of said layer onto a substrate which is maintained at a low temperature, each of said vaporising operations giving rise to the deposition of a stratum forming part of the ultimate layer, the fibres of the structure thus obtained naturally being orientated substantially perpendicularly to the surface of the substrate. The same investigations have shown that, other things being equal, the adhesion is still further improved if the substrate is covered, prior to the deposition of the luminescent layer, with an intermediate porous layer whose nature and thickness will be described in more detail in the examples which will be given below.

This structure, furthermore, exhibits a variety of advantages which will be described in the following.

The present invention relates to an X-ray or gammaray scintillator constituted by an activated luminescent layer whose fundamental constituent is made of alkaline halides and deposited upon a substrate, characterised in that said luminescent layer is essentially consti-' tuted by needles disposed substantially perpendicularly to said substrate.

The invention will be better understood from a consideration of the ensuing description and the attached figures in which 'FIG. 1 is a schematic sectional view illustrating the main elements of an X-ray or gamma-ray image intensifier tube FIG. 2 is a schematic sectional view illustrating the principal elements of an installation employed in the manufacture in accordance with the invention, of the detector screen of the tube shown in FIG. 1

FIG. 3 is a sectional view on an enlarged scale, of part of the detector screen of FIG. 2.

In FIG. 1, which illustrates an example of how the invention may be applied, there can be seen inside an evacuated envelope 1, in the neighbourhood of the extremities thereof, a detector screen 2, a display screen 3 and, between the two, electrodes 4, 5 and 6 for the control and focusing of the electron beam,the latter being symbolised by oblique lines issuing in operation from the screen 2 and directed onto the screen 3.

The voltage sources to which the various electrodes of the tube are connected, have not been shown. The

screen 2 is located in the path of the X-ray or gamma radiation, indicated by arrows, opposite the object 7 whose image is to be displayed on the screen 3.

The detector screen 2 to which the invention is applied, essentially comprises, on a substrate, so called screen substrate, a luminescent layer sensitive to the incident radiation from the object, and a photosensitive layer facing the fluorescent display screen and emitting electrons towards the same as already indicated hereinbefore.

In a variant embodiment, which is not illustrated here, the substrate of the screen 2 is merged into the front face 10 of the envelope 1.

All the features of the invention apply equally to either case.

FIG. 2 illustrates the diagram of an installation for deposition by vaporization under vacuum, this being the process employed for the manufacture, in accordance with the invention, of the detector screens of tubes such as that shown in FIG. 1 1 inside an envelope in which a vacuum is maintained by a pump 21, there are placed a screen substrate in the form of a spherical dish, carrying a lining 32 to which reference will be made hereinafter and whose thickness has been shown very much exagerated, and a vaporiser consisting of a crucible 22 containing the material 24 of which is made the luminescent layer of the ultimate screen, an alkaline halide in fact, surrounded by a heater resistor 23 for producing the requisite vaporising temperature the vaporised material condenses on the screen substrate 30, 32.

The deposition of the luminescent layer 31 (also shown exageratedly thick) takes place in the: form of successive vaporising operations of limited duration, in

order thus to limit the temperature to which the substrate 30, 32 is raised under the effect of radiation from the vaporiser 22, 23 and the condensate. The maximum permissible temperature in accordance with the invention, which the screen substrate should reach during the course of these operations is about 150C. In order to satisfy this condition, there is placed for example upon the screen, in intimate contact therewith, the component having the shape shown at 25 and containing passages 26 through which a cooling fluid is circulated.

By this method, a layer of alkaline halides 31 of f1- brous structure, having the needle form 40 Illustrated at an enlarged scale in FIG. 3, with the needles perpendicular to the surface of the substrate in the direction of the cross-hatching in FIG. 2, is obtained. These needles, which have diameters ranging between 5 and 10 p at their bases, are arranged side by side at the base level and separated by gaps 41 of less than 1 u. (this is proved by microscopic examination). Depending upon the evaporation rate used, their density is between 85 and 95 percent of that of the crude product 24.

The layer 32 is designed to promote the bonding of V the needles 40 to halide layer 31.

It is designed by a method which depends upon the nature of the element of which it is made and upon the nature of the substrate 30. In particular it may itself be produced by vaporising, utilising the same installation.

Hereinafter, by way of example the design characteristics of the screen in accordance with the invention have been listed Luminescent layer or scintillator (31) Screen substrate (30) spherical glass dish, 5/10 mm thick and 160 mm radius.

3 to 4 g of ICs activated by 2 percent of gallium arranged in a tantalum dish 200 mm from the screen substrate.

Vaporization carried out under a vacuum in the order of lO mm/hg.

Temperature of dish maintained at 620C for 2 minutes approximately when the l0 mm/hg vacuum is reached.

three to 10 successive vaporising operations.

Thickness of deposit with each vaporising operation 7 to ll p Intermediate layer ('32) Material nickel Vaporisation under vacuum Thickness 200 to 1,000 nm, deposited in a single stape The bonding of the nickel to the glass substrate 30 can be improved by toughening the glass in a hydrofluoric bath containing 10 percent HF or F HNI-I In the example of FIG. 3, the photo-sensitive layer 34 is only deposited upon the layer of scintillator 31, after the incorporation of the screen (which at this stage comprises the substrate, the intermediate layer and the scintillator layer) in the tube, the latter already being equipped with the display screen and all the other electrodes.

This layer itself is produced by the vaporising of antimony and alkaline metals at a temperature of around 100C.

Of course, various other embodiments are equally possible for example, the substrate 30 could be made of metal instead of glass, for example of aluminum or beryllium and have a thickness in the same order as that of a glass substrate, being covered on that of its faces in contact with the scintillator with an oxide layer 5 to 10 M in thickness obtained in that case by anodising in the case of a glass substrate the intermediate layer 32 may also consist of an alloy of Fe, Ni, Co, already known in the art in the context of electronic tubes by the name of FERNICO of stainless steel or of -20 nickel-chrome with a thickness in the same order as that of the nickel layer of the foregoing example.

The advantages of the detector screens in accordance with the invention, over the prior art ones, are numerous The fibrous structure of the halide layer constituting the scintillator provides mechanical flexibility, enabling this layer to follow distortions of the substrate which are liable to occur during operation which flexibility is very considerable. Because of their fibrous structure, the scintillators in accordance with the invention will in other words tolerate a much larger thickness without deterioration and likewise a greater temperature difference between the assembly of substrate and intermediate layer, 30 32, and the halide layer, than will prior art screens, both these factors being in line with the objectives set out hereinbefore.

Likewise, they will tolerate a much larger gap between the coefficients of expansion of the substrate and that of the material of which the luminescent layer is made thus, they yield a wider choice as concerns the material of which the substrate is made. In particular, with the needle-like halide layers of the invention it is possible to choose substrates of plastic material which have a coefficient of expansion in the order of 1,000 X lO' /C, which value is substantially higher than that of cesium iodide which is in the order of approximately 510 X 10 /C, without any drawbacks being incurred as far as the luminescent layer is concerned. Furthermore, the use of plastic materials has the advantage of enabling large-sized screens to be produced which have a predetermined geometric profile produced by polymerisation or moulding.

In this case, it is necessary, of course, to restrict the selection to plastic materials which have a low vapour tension and do not decompose at the tube stoving temperatures. Polytetrafluorethylene and the silicon resins of type Si 804 and S1 808 produced by Societe Industrielle des Silicones, satisfy these requirements particularly well.

The screens in accordance with the invention, furthermore, provide improved spatial resolution, the light produced by excitation of X-ray or gamma-radiation in the luminescent layer, being for the major part channeled through. the needles, with no surface diffusion.

In the case of the scintillators in image-intensifier tubes as shown in FIG. 1, the design of the photosensitive layer 34 (FIG. 3) of the detector screen is itself facilitated by the structure of the luminescent layer 31, same Figure the photo-sensitive material enshrouds the tips 42 of the needles 40 and this improves its bonding to the luminescent layer 31. The layer 34 lays down onto the top of the needles in the interstices 43 between the needles 40. At the same time, the probability of the collection by the photo-sensitive layer of light photons generated in the luminescent layer, is increased.

The present invention, which has been described in the context of a scintillator forming an integral part of a screen with a photo-emissive photo-sensitive layer, itself incorporated in an X-ray or gamma-ray imageintensifier tube, relates more generally to all devices which incorporate a scintillator in accordance with the invention.

It covers uses of such a scintillator whether incorporated in a tube or not.

In the case where the scintillator is incorporated into a screen which also comprises a photo-sensitive layer, it extends equally to the case where same is photoemissive or photo-conductive.

Finally, there fall within the scope of the invention all X-ray or gamma-ray tubes which, while incorporating a screen comprising a scintillator in accordance with the invention, are designed not for display as in the example described but for the recording or storage of signals produced by said screen, for purposes of ultimate restitution.

In a general way, the invention includes embodiments open to the person skilled in the art, other than those described and illustrated here. All said variant embodiments fall within the scope of the present invention.

What is claimed is 1. An X ray or "y ray scintillator capable of emitting photons within the luminous radiation range when exposed to X or 'y ray comprising a substrate transparent for said X or 'y ray and a luminescent layer deposited onto said substrate, said luminescent layer being made of alkaline halides having a fibrous structure constituted by tapered bodies having their bases applied side by side onto said substrate and extending substantially perpendicularly beyond the surface of said substrate.

2. A scintillator as claimed in claim 1 further comprising between said substrate and said fibrous alkaline halide luminescent layer, an intermediate layer made of a material which is transparent to x-rays or gamma-rays and which facilitates the bonding of said luminescent layer to said substrate.

3. A scintillator as claimed in claim 2, wherein said substrate is made of an insulating material and said intermediate layer is made of a metallic material.

4. A scintillator as claimed in claim 2, wherein said substrate is made of a metallic material and said intermediate layer is made of an insulating material such as an oxide of said metallic material.

5. A detector screen for X or 'y ray image-intensifier tubes, comprising a scintillator in accordance with claim 1 and a photo-sensitive layer deposited onto said luminescent layer of said scintillator.

6. A screen as claimed in claim 5 wherein said photosensitive layer is a photo-emissive layer.

7. A screen as claimed in claim 5 wherein said photosensitive layer is a photo-conductive layer.

8. A method of producing a scintillator capable of emitting photons within the luminous radiation range when exposed to X or y ray comprising a substrate transparentfor said X or y ray and a luminescent layer deposited onto said substrate, said luminescent layer being made of alkaline halides having a fibrous structure constituted by tapered bodies having their bases applied side by side onto said substrate and extending substantially perpendicularly beyond the surface of said substrate consisting in vaporizing said alkaline halide onto said substrate in an evacuated enclosure enclosing said substrate and said alkaline halide to be vaporized, said vaporization being made in several steps while said substrate is permanently cooled by cooling means disposed within said enclosure to provide cooling of the face of said substrate where no alkaline halide is deposited, thus assuming a temperature of said substrate never in excess of C. 

2. A scintillator as claimed in claim 1 further comprising between said substrate and said fibrous alkaline halide luminescent layer, an intermediate layer made of a material which is transparent to x-rays or gamma-rays and which facilitates the bonding of said luminescent layer to said substrate.
 3. A scintillator as claimed in claim 2, wherein said substrate is made of an insulating material and said intermediate layer is made of a metallic material.
 4. A scintillator as claimed in claim 2, wherein said substrate is made of a metallic material and said intermediate layer is made of an insulating material such as an oxide of said metallic material.
 5. A detector screen for X or gamma ray image-intensifier tubes, comprising a scintillator in accordance with claim 1 and a photo-sensitive layer deposited onto said luminescent layer of said scintillator.
 6. A screen as claimed in claim 5 wherein said photo-sensitive layer is a photo-emissive layer.
 7. A screen as claimed in claim 5 wherein said photosensitive layer is a photo-conductive layer.
 8. A method of producing a scintillator capable of emitting photons within the luminous radiation range when exposed to X or gamma ray comprising a substrate transparent for said X or gamma ray and a luminescent layer deposited onto said substrate, said luminescent layer being made of alkaline halides having a fibrous structure constituted by tapered bodies having their bases applied side by side onto said substrate and extending substantially perpendicularly beyond the surface of said substrate consisting in vaporizing said alkaline halide onto said substrate in an evacuated enclosure enclosing said substrate and said alkaline halide to be vaporized, said vaporization being made in several steps while said substrate is permanently cooled by cooling means disposed within said enclosure to provide cooling of the face of said substrate where no alkaline halide is deposited, thus assuming a temperature of said substrate never in excess of 150*C. 